decodeString: reuse CharsetDecoder
decodeString: reuse CharsetDecoder

Avoids ThreadLocal lookups in Charset#decode, another ~5% improvement
on Benchmark.

maxmind/MaxMind-DB-Reader-java,phraktle/MaxMind-DB-Reader-java,maxmind/MaxMind-DB-Reader-java,phraktle/MaxMind-DB-Reader-java